External camera laggy - low frame rate

I've just upgraded from a 2018 MacBook Pro to the MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports).

When using the Mac's FaceTime camera, everything works fine. However, when connecting it to my LG UltraFine 5K (as I was my previous Mac), the camera feed is laggy (in every application).

The frame rate is really low and erratic - ranging from one frame per second to even fewer.

My previous Mac worked fine with the same monitor.

I've tried:

Using a different Thunderbolt port on the Mac to connect to the display.

Resetting the Mac's PRAM and SMC.

Creating and testing with a new user.

Reinstalling 10.15.4.

Checking for firmware updates for the external display.

all to no avail.

I can't seem to fix the issue, nor can I find any suitable help online.

Any thoughts are appreciated.